In order to satisfy ANSI requirements for digital data communications, telephone subscriber copper wire lines have had to comply with specified industry standard performance criteria, that inherently limit the operational range of a two-wire loop. For example, in the case of ISDN basic rate digital subscriber lines (having a data rate of 160 kilobits per second, including bidirectional data payload and overhead maintenance channels), the ANSI standard T1.601 for two-binary/one, quaternary/four level (2B1Q) modulation, two-wire, full-duplex data transport with echo cancellation, requires that the two-wire loop loss of the ISDN channel not exceed 42 dB at 40 KHz, or 1300 ohms, resistive. This loss limitation necessarily constrains the operational range of a 2B1Q ISDN two-wire loop to a range on the order of 15-18 kft, using No. 26 (American Wire Gauge) wire, and commercially available ISDN transceiver interface equipment.
Extending ISDN communications to digital communication equipment at a customer premises that lies geographically beyond this range requires that the service provider either install repeaters, or use a different communication medium, such as a T1 carrier fiber optic link. Unfortunately, each of these proposed solutions to the extended range problem carries with it a substantial cost penalty that the customer is unwilling to bear. The repeater approach is especially costwise egregious, as it requires installation of both an office end repeater powering unit, plus a repeater mounting pole, or a subterranean, environmentally hardened housing (bell jar) for the repeater. Not only does this involve the use of additional equipment (including the cost of the repeater hardware and its installation), but it entails the expense and labor of maintaining the repeater enclosure.
Similarly, although T1 channel banks located in both the switch office and another downstream location (office or remote hut), that is geographically `close` to the subscriber premises, can accept basic rate interface transmission extender (BRITE) cards for T1 carrier extension, the fact that T1 carrier systems have a capacity for `multiple` extended basic rate service means that their use to deliver only a single basic rate extended service is prohibitively expensive and impractical.
Advantageously, these problems are solved by the invention described in U.S. Pat. No. 5,809,033 (hereinafter referred to as the '033 patent), issued Sep. 15, 1998 to M. Turner et al, entitled: "Use of Modified Line Encoding and Low Signal-to-Noise Ratio Based Signal Processing to Extend Range of Digital Data Transmission Over Repeaterless Two-Wire Telephone Line," assigned to the assignee of the present application and the disclosure of which is incorporated herein. Pursuant to the invention described in the '033 patent, the range of digital data communication services, such as a basic rate ISDN channel, is `repeaterlessly` extendable to well beyond the basic rate ISDN two-wire loop range, by a communication mechanism which changes the line code or modulation format, and employs enhanced signal processing techniques, which may be of the type employed in high bit rate digital subscriber line systems, to accommodate a diminished signal-to-noise ratio resulting from the added insertion loss inherent in the extended transport distance of the two-wire pair.
Now although the digital telecommunication range extension scheme described in the '033 patent enables the telecommunication service provider (TELCO) to deliver two different types of digital communication signals (basic range and extended range digital signals) to its customers, from a hardware inventory and delivery standpoint, the TELCO prefers to stock and install only one type of digital communication equipment, and desires that what is installed be the correct equipment for the line of interest. This is particularly true, for example, in the case of ISDN service where the customer premises equipment is located at the outer edge of the basic rate (ISDN) service range.